Sanyam Jain, Academic Content Creator at Edumarz
Plants, like animals, also use electro-chemical signals to communicate information but, do not have a specialised nervous system or muscular system.
They respond to environmental stimuli through movements which movements could be either i) growth-dependent or ii) growth-independent
Growth-dependent movements (tropic movements)
Tropic movements are directional but slow.
In tropic movements, a change in cellular growth induced by the stimulus occurs.
Phototropism
Movements of plant parts in response to sunlight
Roots are negatively phototropic i.e. they bend away from the sunlight
The shoot is positively phototropic i.e. it bends towards the sunlight
Hydrotropism
Bending of roots towards higher water (or moisture) content in the soil
Geotropism
Movement of plant parts in response to gravity
Roots are positively geotropic i.e. they bend in the direction of gravity
The shoot is negatively geotropic i.e. it bends in the direction opposite to gravity
Chemotropism
Movement of plant parts induced by some chemical
Example– Growth of pollen tube towards ovules inside carpel
Thigmotropism
Movement of plant parts in response to touch
Example– Growth of pea tendril upon contact with any support
Growth-independent movements (nastic movements)
These are non-directional, quick movements in plant parts caused by changing water content in the cells resulting in a change in the cell shape.
Example– Bending of leaves of Mimosa pudica (touch-me-not) on touch
Different plant hormones help to coordinate growth, development and responses to the environment.
They are produced at sites different from where they act and simply diffuse to the area of action.
Auxin
Growth promoting hormone that helps the cells to grow longer
Produced in the actively growing shoot tip
Diffuses to the side opposite to that of sunlight causing increased cell growth on that side of stem resulting in bending of the shoot towards sunlight
Gibberellins
Cause stem elongation
Cytokinins
Enhance plant growth by promoting cell division
Present in greater concentration in areas of rapid cell division, such as in fruits and seeds
Abscisic acid
Only growth inhibitor hormone
Cause leaf wilting & closing of stomata